Good vs. bad BMS?

[KevinC_63559]

Still new with LiFePO4 batteries, but aware that the typical 12V system uses 4 cells, and that a BMS is highly recommended/required to, at least, keep the cells balanced.

Trying to understand what makes a good one, vs a bad one. To illustrate, lets compare two units. Our host 4 cell balancer and monitor, presumably the best available:

• LED indicates FULL / Balancing is active
• 3.55 to 3.65 volts Dynamic balancing
• 310 mA balance current at top of charge 3.65 Volts
• Very low battery drain 170 uA below 3.55 volts
• For 12, 24, 36 and 48 volt Battery Packs using 1 to 4 units
• Reversed cell and overtemperature foldback protection
• High accuracy +/- 0.01 volts typical +/-0.02 Maximum
• All circuits are optically isolated including DATA
• ESD Protection on all lines
• 4KV Optical Isolation between cells and DATA
• LED Blink Warning indicates a cell under 3 volts
• LED Fast Off Blink indicates cell overvoltage 3.75 volts

With the notes:

Optional DATA connection to system master unit for monitoring cell voltages from 2.50 to 4.50 volts

System Master will add low cell and high cell voltage protection (BMS Function) and also faster dynamic balancing
vs. the stats on the BMS built into my super-cheap ($150 12V100A ), Amazon sourced "educational" Fiune brand battery:

100A BMS with low temperature protection
Upgraded 100A BMS has anti-overcharge, over-discharge, overcurrent, and short circuit, allowing the lifepo4 battery 100ah to have a strong and stable performance in each cycle. A built-in thermal cutoff prevents charging above 140°F (60°C) and enables low-temperature shutdown protection for extreme temperatures below 32°F/0°C. It protects the battery from freezing temperatures for longer battery life.
Note the Fiune BMS supports bluetooth with an IOS app that shows State of Charge as a percentage, remaining capacity in amps, a few on/off states such as ChgMos, DisMos, Balance, and Protection. Also 2 decimal place readings: TotalVolt 12.66V, Current 0.00A, Power 0.00W, VolHigh 3.42V, VolLow 3.41V, VolDiff 0.01. AveVol 3.42, Cycle-Index 1, Mos in C and F, T1 in C and F, and all 4 cell single voltages 3.24, 3.42, 3.42, 3.41 in color codes of green, blue and grey all real-time. Additionally there is a control panel with on/off Charge Switch, Discharge Switch, and buttons for AutoBalance, Current calibration, Voltage calibration, ClearWarning, and Reset capacity.

Too me, this looks like comparing apples to oranges, but both are "BMS". I have no idea how to tell is one is "better" than the other.

I did a quick search hoping to find some description of fields (I have no idea what Mos and T1 are, or what the color codes are suppose to mean) on the XiaXiang BMS app. So far, the only thing I learned is that they private label their products, with examples being Jiabvaida BMS and JBD BMS) and that their bluetooth module cost all of $6.98.

Lol - did another search and found this: https://diysolarforum.com/threads/jbd-xiaoxiang-bms-apps-list.29890/ That post shows the screens I outlined above.

Also found the Overkill Solar IOS app, which is a bit clearer, and quickly found the battery (once I shutdown the other app). In fact, Overkill Solar shows voltages to 3 decimal places. Overkill Solar also has a user manual that is downloadable, about to read that.

But back to the main question: How do I tell a "good" vs. a "bad" BMS prior to something like a catastrophic failure?

 

[sunshine_eggo]

But back to the main question: How do I tell a "good" vs. a "bad" BMS prior to something like a catastrophic failure?

You test its limits. Deliberately overcharge and over-discharge to confirm cutoff works. If it's a smart BMS where you can change the settings, you don't even need to exceed actual limits... just move the voltages much closer.

 

[Rocketman]

Another way - lazy man way…

Find out the reputation from reading way too much, buy one that has a good reputation without a lot of bad points and put it in.

I kinda did that when making a small 12v 50ah portable battery- I put an Overkill bms in - which is the JBD. It has been working fine for me… but that battery probably only has 3 or 4 cycles on it in 3 years. (My need for it changed shortly after I got it made), but it was a great learning process.

 

[KevinC_63559]

You test its limits. Deliberately overcharge and over-discharge to confirm cutoff works. If it's a smart BMS where you can change the settings, you don't even need to exceed actual limits... just move the voltages much closer.

That, of course, requires buying one of everything under consideration. Very Effective, but not ideal for the budget. Still, appreciate the response.

 

[sunshine_eggo]

That, of course, requires buying one of everything under consideration. Very Effective, but not ideal for the budget. Still, appreciate the response.

Let's review. Your question:

But back to the main question: How do I tell a "good" vs. a "bad" BMS prior to something like a catastrophic failure?

I answered your question. You did not ask "prior to something like purchase?"

 

[KevinC_63559]

Let's review. Your question:



I answered your question. You did not ask "prior to something like purchase?"

Fair enough. Please amend my question to be "Prior to something like purchase".

 

[sunshine_eggo]

Fair enough. Please amend my question to be "Prior to something like purchase".

You're over-analyzing the shit out of it.

Find a brand that has a good reputation with the features you need, and then buy it from a reputable supplier.

Minimum features required of a LFP BMS to protect the cells/battery:

1. High voltage cell protection
2. Low voltage cell protection
3. Low temperature charge protection
4. Low temperature discharge protection
5. Short circuit protection
6. Over-current protection
7. Passive balance

Anything else is an upgrade or a bonus.
Anything that doesn't meet the above should be discarded.
If you're reading a wall of Chinglish Technobabble, move on from that listing.

MOS may refer to MOSFET. Also referred to as FETs. They are basically the electronic switches that do the shutting off/turning on.

T1 is the first temperature sensor. It may refer to the BMS internal temperature sensor, or it may refer to the temperature sensor attached to measure the battery temperature.

3 decimal places is mostly noise and .XX is more than adequate.